1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to an improved apparatus for imparting vibrational energy to the earth wherein selected combinations of compressional and shear wave energy can be generated simultaneously.
2. Description of the Prior Art
Recent advances in the ability of seismic data acquisition and processing equipment to handle large amounts of data have stimulated greater interest in the development of versatile high energy seismic sources adapted to produce both compressional and shear waves. One goal in the development of "multi-component" seismic sources is to enable the operator to more efficiently measure subsurface formation properties such as porosity, fracture orientation, pore fluid type or lithologic type. It is known that P, SH and SV waves travel with their own distinct velocities when passing through a particular geologic formation. Thus, changes in formation properties, such as rock type, porosity and the like may be detectable by changes in the full wave field (i.e., the P, SV and SH wave velocities). A strong correlation observed between lateral variations and the compressional to shear wave velocity ratio (V.sub.p /V.sub.s) with known changes in a geologic formation suggests that these variations may be used to infer changes in the lithology, porosity or pore fluid content of a subsurface formation. By recording the full wave form of a seismic wave, including P, SV and SH waves, one can get far more detailed information concerning rock properties at depth than would otherwise be possible.
In applicant's co-pending commonly assigned U.S. patent application, Ser. No. 792,531 filed on Oct. 29, 1985, in the name of Tom P. Airhart entitled "Full Wave Field Vibratory Seismic Source", there is described an apparatus for coupling vibratory energy into the earth at a given seismic source point. As described in the subject application, a hydraulic vibrator is mounted on an earth-contacting base plate by means of a fixture which enables a rotation of the vibrator axis about two mutually perpendicular axes. Rotation about these two axes permits the operator to position the vibrator axis in any desired direction with respect to a seismic survey line. Energizing the vibrator then transfers energy through the mounting fixture to the base plate and thence into the ground in the preselected direction. Orthogonal 3-element geophones positioned remotely from the seismic source point along the seismic survey line may receive and record simultaneously the resultant P, SH and SV waves, thus enabling full wave field analysis of the sub-surface reflections.